In a general rubber devulcanization, the process involves cleaving of the monosulfidic, disulfidic and polysulfidic crosslinks (carbon-sulphur or sulphur-sulphur bonds) existing in a vulcanized rubber. Technologies that were developed for this purposeful process, typically adopt techniques involving chemical, ultrasonic, microwave or biological devulcanization.
It is known that in chemical devulcanization, the process makes use of materials such as organic solvent, oil and chemicals, or organic compounds for devulcanizing rubber. Ultrasonic devulcanization and microwave devulcanization, on the other hand, make use of ultrasonic waves and microwave heat to devulcanize the rubber, and biological devulcanization is carried out through microbial attacks on the sulphur crosslinks of the vulcanized rubber so as to break the bonds.
There are several prior arts divulged the methods for producing devulcanized rubber. U.S. Pat. No. 5,770,632 discloses a method of reclaiming elastomeric material, or known as De-Link process, through the use of a chemical mixture that is capable of initiating proton exchange at a controlled temperature. The prior art discloses a method for producing devulcanized rubber via vigorous mixing of the rubber powder by stirring prior to the addition of the chemical composition and processing in a roll mill.
European Patent No. 1 242 520 B1 owned by Levgum, Ltd. discloses a modifier for devulcanizing cured elastomer and a method to devulcanize rubber using said modifier. As discussed in the prior art, the cured elastomers are brought to a stressed polymer structure state and then treated by organic cations that are generated by the modifier as claimed.
U.S. patent application Ser. No. 11/636,611 owned by The SF Materials Corporation discloses a method of devulcanizing rubber by contacting the vulcanized rubber with turpentine liquid in a reaction mixture in the absence of an alkali metal. The reaction mixture further comprises a solvent or a liquid that is immiscible with the turpentine liquid. The vulcanized rubber is contacted with the turpentine liquid in the presence of an energy input comprising thermal energy, microwave energy, ultrasonic energy, mechanical shear-forces or mixtures thereof.
U.S. Pat. No. 4,161,464 owned by BF Goodrich Company describes a method of devulcanizing rubber through a phase transfer chemical reaction catalysed with certain onium salts that allows the transport of hydroxide ions into vulcanized rubber particles to selectively cleave polysulfide crosslinks for producing a recyclable devulcanized rubber. The devulcanizing process involves the steps of swelling vulcanized rubber particles in an organic solvent having dissolved therein an onium salt, and thereon contacting the swollen vulcanized particles with an aqueous solution of a base to provide sufficient hydroxide ions for the phase transfer reaction.
U.S. Pat. No. 6,387,966 B1 describes a method of devulcanizing waste rubber by adding a modifying composition to waste rubber particles and subsequently crushing the waste rubber particles through a roll mill thereby creating modified crushed particles. The modification of the rubber particles involves the breaking of at least 70% sulphur to sulphur bonds and no more than 10 to 15% carbon to carbon bonds.
U.S. Pat. No. 7,189,762 B2 discloses a method for modifying crosslinked rubber. The method for modifying crosslinked rubber is carried out by using mechanical elongational and shear force in the presence of carbon dioxide in a supercritical fluid state.
PCT Patent Application No. PCT/CA2011/000285 relates to a method of regenerating vulcanized crumb rubber comprising the steps of mixing the crumb rubber and a lubricant at room temperature, transferring the mixture to a thermokinetic mixer having a rotor shaft containing blades to increase temperature of the mixture until a devulcanizing temperature is reached, reducing the temperature of the mixture and recovering the regenerated rubber. In the prior art, the method of regenerating vulcanized crumb rubber utilizes higher range of devulcanizing temperature, hence this requires increased energy consumption.
The conventional rubber devulcanizing technologies as disclosed in preceding prior art documents are mostly impractical to produce a viable substitute to virgin rubber due to the limitation of scalability to large volume production. Other problems encountered by the conventional recycling technologies are intensive energy consumption, poor cure safety, poor material compatibility during processing, obtention of undesirable product properties and performance, as well as exhibiting poor shelf life.